1. Field of the Invention
The present invention generally relates to metallic covers, and more particularly to a metallic cover used for electronic devices and a method for making the metallic cover.
2. Discussion of the Related Art
Generally, a metallic cover has a more appealing appearance and a better surface feel than a plastic cover, thus metallic covers are popularly used for electronic devices such as mobile phones.
Referring to FIG. 14, a typical metallic cover 10 is shown. The metallic cover 10 includes a rectangular bottom base 11, a first side wall 12, a second side wall 13, a third side wall 14, and a fourth side wall 15. The side walls 12, 13, 14, 15 perpendicularly extend from a periphery of the rectangular bottom base 11. Each of the side walls 12, 13, 14, 15 connects to its adjacent side walls, thus the side walls 12, 13, 14, 15 cooperatively define a cavity (not labeled) for receiving electronic components (not shown).
Each of the side walls 12, 13, 14, 15 and the bottom base 11 are connected at an edge (not labeled). The edge is generally a rounded edge so that the metallic cover 10 can easily be made by metal drawing method.
However, in order to obtain a different appearance, an edge of another typical cover for connecting the side walls and the bottom base may be a beveled edge instead of the rounded edge. Generally, it is practically impossible to produce the beveled edge using the metal drawing method. A typical method for making a metallic cover with a beveled edge includes two following steps: drawing a metal sheet into a preformed cover; pressing the preformed cover into a metallic cover with a beveled edge by a forming die. However, the edges of the metallic cover are prone to cracks, when using the above method, thus decreasing quality of the metallic covers.
In addition, in order to reduce the volume, size or weight of these electronic devices such as mobile phones, the described metallic covers are generally made by thin metal sheet. However, a thickness of the side walls of the metallic covers is the same as that of the bottom base of the metallic covers. As a result, the strength of the side walls of the metallic covers is relatively low. To enhance the strength of the side walls of a metallic cover, a thickness of each side wall of the metallic cover should be greater than that of the bottom base of the metallic cover. It is also practically impossible to produce an unequal-thickness metallic cover using the metal drawing method. The unequal-thickness metallic cover is generally made by a die-casting method. However, the appearance of the unequal-thickness metallic cover made by the die-casting method is not good, thus the unequal-thickness metallic cover needs to be polished to improve the quality of the appearance of the unequal-thickness metallic cover.
At present, unequal-thickness metallic covers can also be manufactured by computerized numerical control (CNC) milling machines. A first typical method for making an unequal-thickness metallic cover will now be described. A relatively thick raw metallic block is provided. The raw metallic block has to go through several milling processes so as to get a preformed body. The preformed body is made into a metallic cover by a process of finish machining. The above method for making unequal-thickness metallic covers takes a great deal of time, for example, it needs more than ten hours for a process to make one unequal-thickness metallic cover of a mobile phone. Because the unequal-thickness metallic cover goes through several milling processes, the efficiency of the described method is low, thereby increasing the cost of the unequal-thickness metallic covers.
Referring to FIG. 15, a metallic cover made of aluminum alloy, e.g., an aluminum alloy of grade AL5052 using the above milling method shows a metallographic photograph of a portion of the side walls of the metallic cover under 23±5 Celsius degrees (° C.) and 40-80% relative humidity (RH) conditions. The internal structure of the metallic cover is magnified by 200×. The metallographic structure of the metallic cover is relatively incompact. A Vickers hardness of the metallic cover under 23±5° C., 40-80% RH and 0.5 kilograms (kgs) is in a range from 68 to 70.
Therefore, a new metallic cover is desired in order to overcome the above described shortcomings. A new method having high efficiency for making such metallic cover is also needed.